It is well known that the methine chains of methine compounds may be crosslinked to improve the stability of the methine compounds in solutions.
Conventional crosslinked methine compounds will be illustrated in detail in comparison with the methine compounds of the present invention hereinafter.
Further, it is conventional and well known in the art that sensitizing dyes are added to silver halide emulsions in the preparation of silver halide light-sensitive materials to enlarge the light-sensitive wavelength regions of the silver halide emulsions and to thereby optically sensitize the silver halide emulsions.
Many compounds are known as spectral sensitizing dyes which are used in the above-described sensitization of silver halide emulsions. Examples of such sensitizing dyes include cyanine dyes, merocyanine dyes and xanthene dyes as described in T. H. James, The Theory of the Photographic Process, third edition, pp. 198-228, 1966 (Macmillan New York).
When these sensitizing dyes are applied to the silver halide emulsions, not only are the light-sensitive wavelength regions of the silver halide emulsions enlarged, but the following conditions must also be met:
(1) The spectral sensitizing regions must be proper.
(2) The sensitizing efficiency must be good and sufficiently high sensitivity must be obtained.
(3) Fogging must not be caused.
(4) Changes in temperature must not result in significant variations in sensitivity.
(5) The sensitizing dyes must not have any adverse interactions with other additives such as stabilizers, anti-fogging agents, coating aids, color formers, etc.
(6) The sensitivity must not fluctuate when silver halide emulsions containing the sensitizing dyes are stored, particularly when the emulsions are stored under high temperature and humidity conditions.
(7) The sensitizing dyes when added to a particular layer must not diffuse to other light-sensitive layers, because such diffusion can result in color turbidity (color mixing) after processing.
The above-described conditions are important factors in the preparation of silver halide emulsions of silver halide photographic materials. Particularly, it has been highly demanded to meet the above-described conditions (2) and (6) with regard to high sensitivity of silver halides and the improvement of stability of raw samples during storage.